1. Field of the Invention
This invention relates to a nonvolatile semiconductor memory device, and more particularly to a nonvolatile semiconductor memory device with electrically rewritable memory cells.
2. Description of the Related Art
With the recent rapid spread of digital cameras, portable audio players, and the like, the demand for high-capacity nonvolatile semiconductor memories has been expanding. NAND flash memories have been widely used as nonvolatile semiconductor memories. To realize a high-capacity NAND flash memory, a multilevel NAND flash memory which stores a plurality of items of data in a single memory cell has been proposed.
In a NAND flash memory, data is determined by the threshold voltage of a memory cell. Accordingly, to record multilevel data, one of a plurality of threshold voltages is set in a memory cell. As elements have been miniaturized further, the distance between memory cells has been decreasing. Consequently, the effect of intercell interference caused by capacitive coupling between adjacent cells has been becoming greater.
In addition, as a related technique, the technique for narrowing the width of the threshold voltage distribution of memory cells by changing the transfer voltage to a bit line in a write operation has been disclosed (Jpn. Pat. Appln. KOKAI Publication No. 2006-331618).